1. Field of the Invention
The present invention relates to a beam recording apparatus of the type in which a plural number of beams are modulated by modulation signals and given information is recorded on a recording medium by scanning the recording medium with the modulated beams.
2. Description of the Prior Art
In the conventional beam recording apparatus a simple beam has been used to record information on a recording medium. In order to increase the recording speed of such apparatus, it is required to increase the transfer speed of a beam modulating information signal.
Also, it is required to increase the speed of principal scanning. When a rotary polygonal mirror is used for principal scanning, the rotational speed of the mirror must be increased, sometimes, up to tens of thousands of rpm. Due to structural limitation, such high rotational speed is difficult to attain. For these reasons, it has been practically impossible to provide a high speed recording apparatus using a single beam.
As a solution to the above problem, it has already been proposed to use a plural number of beams. In the known multibeam recording apparatus, a plural number of beams are independently modulated by recording signals and a recording medium is scanned by these modulated beams at the same time.
An important problem involved in the multibeam recording system is the manner of arrangement of the beams relative to the scan direction, which will be described hereinunder with reference to FIGS. 1A and 1B.
FIG. 1A illustrates the case wherein beams B1-B4 are arranged at right angles with the scan direction SL. In this case, the positions irradiated by the beams are all concentric relative to the scan direction. Therefore, all of the beam positions can be detected by detecting only one of the beam positions. Consequently, controls on the plural number of beams including the control of start timing of beam modulation can be carried out without difficulty.
However, this arrangement of beams at right angles with the scan direction SL has a disadvantage that the spacing between picture elements formed on the recording medium is determined directly by the spacing between scanning beams. The beam spacing should be equal to the picture element spacing. As the spacing between scanning beams is limited, the density of picture elements is automatically limited by the limitation imposed on the arrangement of scanning beams. With this arrangement of beams, therefore, it is practically impossible to reproduce a high density image.
FIG. 1B illustrates another case wherein the beams B1-B4 are arranged at an inclination angle of .theta. with the straight line L-L' normal to the scan direction SL. With this arrangement, it is made possible to obtain a larger beam spacing Pl than the determined picture element spacing PS. This means that the picture element spacing PS can be selected at will independently of the beam spacing Pl. However, in this case, the beam irradiation positions relative to the scan direction are different from each other and therefore different scanning start positions for different beams must be set.
To this end, it has been proposed to use a beam detector and timers. The beam detector is provided at a position out of the range of the recording medium. Prior to scanning, the first beam is directed to the beam detector to obtain a signal. The signal thus obtained is used as a scanning start signal for the first beam. Scanning start signals for other beams subsequent to the first one are obtained by the timers which respectively are actuated by the scanning start signal for the first beam. This method for generating different scanning start signals is the subject of our prior applications (U.S. patent application Ser. Nos. 184,146 and 164,522 the latter having been abandoned in favor of U.S. Continuation Application Ser. No. 395,183).
Naturally, the accuracy of such scanning start signals depends on the precision of the timers used. There may occur such case where the timers set false scanning start positions the scanning beams. Generally speaking, this method lacks adequate reliability.
Also, if there occurred drop in output or failure of proper output in any one of the plural number of scanning beams, then an error in operation would result. To prevent such trouble, many detectors must be provided to continuously detect the states of the respective beam generators and also detect the states of outputs from the respective beam generators. This makes the structure of the apparatus very complicated.